Approximately 8% of the genomes of mammals, including humans and mice, are comprised of retroviral elements acquired by infection of germ line cells during the course of evolution. Retroviral insertions in our genome number about 40,000 and are in the same range as the total number of genes encoded by our DNA. Many endogenous retrovirus elements are defective, however some appear to be intact, and several contain one or more viral genes that are expressed during development and certain physiological or pathological conditions. Little is known about the control of retrovirus expression or the influence of such expression on the physiology or pathology of the host. An extensively investigated group of endogenous retroviruses are those giving rise to polytropic murine leukemia viruses (MuLVs) in mice. In several instances polytropic MuLVs have been directly implicated in pathogenesis, including the induction of proliferative, immunological, and neurological disorders. Polytropic MuLVs are formed by recombination of exogenous ecotropic MuLVs with endogenous envelope sequences present in the genomes of inbred mouse strains. Recombination between the exogenous and endogenous retrovirus genomes requires transcription of a complete endogenous provirus to an RNA strand which is co-packaged with an exogenous MuLV transcript as a heterodimeric virion RNA. Upon subsequent infection, the heterodimer can undergo recombination during reverse transcription. Although the endogenous polytropic proviruses are transcribed; replication of the endogenous polytropic viruses in the absence of recombination has not been observed. This may, in many cases, reflect defects such as point mutations or deletions in the endogenous viral genome but may also be influenced by the activity of various restriction factors. The fact that exogenous MuLVs are capable of replicating in mice indicates that they have evolved mechanisms to circumvent the activity of at least some of the restriction factors such as the murine APOBEC3. Thus, exogenous retroviruses might facilitate through complementation, active replication of endogenous retroviruses. We have found that infection of mice by an exogenous virus results in the infectious transfer of complete endogenous proviral genetic sequences. This includes proviruses which are severely defective and possess large deletions as well as proviruses that are full-length. Furthermore, the transferred sequences are transcribed and packaged into virions released from the newly infected cells. At early times after infection with the Friend MuLV, packaging and transfer of intact endogenous retroviruses is much more prevalent than recombination. The mobilization of intact endogenous retroviruses is unprecedented and may have important implications for the involvement of endogenous retroviruses in disease processes.